BIOL 112- After Midterm

Genes and Chromosomes

Genes are physical entities carried by chromosomes, which are made of DNA, a double helix structure that contains the instructions for the development, functioning, growth, and reproduction of all known organisms and many viruses. Chromosomes are tightly packed structures where DNA is coiled around histone proteins, allowing for efficient packaging within the cell nucleus.

DNA purification involves:

  • Grinding up the organism to break down cell membranes and access the genetic material.

  • Extracting lipids and proteins using organic solvents, such as phenol or chloroform, to separate DNA from other cellular components.

  • Precipitating DNA using ethanol or isopropanol, which helps isolate DNA from the solution by making it less soluble.

Historical Discoveries

Friedrich Miescher: Isolated "nuclein" (now known as DNA) from cell nuclei in pus, marking the first discovery of nucleic acids, which laid the groundwork for genetic research.

Frederick Griffith: Conducted pivotal experiments with the bacterium Streptococcus pneumoniae, discovering a "transforming principle" that enabled non-virulent R strains to become virulent S strains through the uptake of genetic material from dead S strains. This transformation was crucial in supporting the concept that DNA carries genetic information.

Avery, MacLeod, and McCarty's Experiment: Provided compelling evidence that DNA is the substance responsible for heredity by demonstrating that the transforming principle identified by Griffith was indeed DNA through a series of controlled experiments.

Hershey-Chase Experiment: This landmark experiment involved using bacteriophages labeled with radioactive isotopes, revealing that only DNA entered bacterial cells during infection, conclusively determining that DNA, not protein, is the carrier of genetic information.

Structure of DNA

DNA is composed of four types of nucleotides: adenine (A), thymine (T), guanine (G), and cytosine (C). This sequence of nucleotides encodes genetic instructions through the formation of a double helix structure stabilized by hydrogen bonds between complementary bases:

  • A pairs with T through two hydrogen bonds.

  • G pairs with C through three hydrogen bonds, making this pairing more stable.

The double helical structure features major and minor grooves, which serve as binding sites for proteins that regulate transcription and replication.
Chargaff's Rules: The ratios of nucleotides follow specific patterns: the amount of adenine should equal thymine, and the amount of guanine should equal cytosine in a DNA molecule.

Watson and Crick's Model: Proposed that DNA exists as a double helix formed by two antiparallel strands running in opposite directions, with the phosphate backbone on the outside and the bases on the interior.

DNA Replication Models:

  1. Conservative: Original strands remain together after replication, producing one entirely new double helix.

  2. Semiconservative: Each new DNA molecule consists of one original and one newly synthesized strand, a model confirmed by the Meselson-Stahl experiment.

  3. Dispersive: Chains of DNA break into segments, with new and old DNA interspersed throughout each strand.

DNA Replication Process

Steps of DNA Replication:

  1. Helicase unwinds the DNA strands at the replication fork, creating two single-stranded templates.

  2. Primase synthesizes short RNA primers complementary to the DNA template, necessary for DNA polymerases to initiate synthesis.

  3. DNA Polymerase III extends the new DNA strand by adding nucleotides in the 5' to 3' direction, synthesizing the leading strand continuously toward the fork.

  4. Lagging Strand: Due to the antiparallel nature of DNA, this strand is synthesized discontinuously in small segments known as Okazaki fragments, which are later connected.

  5. DNA Polymerase I removes RNA primers and replaces them with DNA.

  6. DNA Ligase joins the Okazaki fragments together, sealing any gaps in the sugar-phosphate backbone.

  7. Topoisomerase alleviates the torsional strain ahead of the replication fork.

  8. Single-Stranded Binding Proteins (SSBs): Prevent the separated strands from re-annealing, stabilizing the replication fork.

  9. Telomerase: Extends telomeres in eukaryotic cells, allowing for the complete replication of linear chromosomes without losing important genetic information at the ends.

Transcription and Translation

Genetic Code: The sequence of nucleotide bases (A, T, C, G) in DNA encodes genetic instructions that dictate cellular functions.
Each set of three nucleotides, known as a codon, corresponds to a specific amino acid (e.g., the codon UUU codes for phenylalanine).

Garrod's Experiment: Investigated alkaptonuria, demonstrating that metabolic disorders may arise from missing enzymes due to specific gene mutations.

Beadle and Tatum: Advanced the one gene-one enzyme hypothesis, establishing a link between specific genes and the production of enzymes.

Central Dogma of Molecular Biology

The central dogma describes the flow of genetic information within a biological system: DNA transcribes into RNA, which is then translated into proteins.

  • Messenger RNA (mRNA): Transmits genetic information from DNA to ribosomes, where proteins are synthesized.

  • Transcription: RNA polymerase synthesizes mRNA from a DNA template, catalyzing the formation of RNA nucleotides complementary to the DNA strand.

  • Translation: Ribosomes decode mRNA codons, assembling associated amino acids into polypeptide chains. Transfer RNA (tRNA) plays a critical role by bringing amino acids to ribosomes and pairing anticodons with corresponding mRNA codons.

Mutations:
Mutations are alterations in DNA sequences that can lead to changes in the resulting proteins.

  1. Point Mutations: Classified into three types: silent (no change in amino acid), missense (changes one amino acid), or nonsense (introduces a premature stop codon).

  2. Frameshift Mutations: Caused by the insertion or deletion of nucleotides, disrupting the set reading frame and potentially leading to significant changes in the resultant protein.

Viral Structure and Life Cycle

Viruses: These microscopic infectious agents require host cells for replication and are composed of nucleic acid (either DNA or RNA) surrounded by protein coats known as capsids, with some viruses possessing lipid envelopes that aid in cell infection.

Bacteriophages: A specific type of virus that infects bacteria, serving as a model for studying viral mechanisms.

Types of Viral Life Cycles:

  • Lytic Cycle: Characterized by the rapid hijacking of host cellular machinery to produce more viral particles, culminating in the lysis (or bursting) of the host cell.

  • Lysogenic Cycle: In this cycle, viral DNA integrates into the host's chromosome, remaining dormant (as a prophage) until it becomes activated, often in response to environmental triggers.

Genetic Engineering and Applications

Genetic Engineering: Involves modifying an organism's DNA to achieve desired traits, commonly achieved through recombinant DNA technology and gene cloning techniques.

  • Insulin Production: An important application where the human insulin gene is inserted into bacterial plasmids, allowing bacteria to produce insulin for diabetes treatment on a mass scale.

  • Gene Cloning Process: Involves locating the gene of interest (e.g., insulin), preparing it for insertion, incorporating it into plasmids, and transforming the bacteria to express the gene of interest.

Cystic Fibrosis (CF): A recessive Mendelian disorder resulting from mutations in the gene encoding the cystic fibrosis transmembrane conductance regulator (CFTR), crucial for chloride ion transport, leading to severe respiratory issues and mucus buildup.
Genetic Screening: A vital tool for identifying carriers and individuals affected by genetic disorders; gene therapy aims to correct the underlying genetic defects in specific cell types, such as lung cells, to alleviate symptoms.

Restriction Fragment Length Polymorphisms (RFLPs): RFLPs are variations in DNA sequences that result in different lengths of restriction enzyme digestion products. These variations can be utilized in genetic mapping, parental testing, and forensic science to differentiate between individuals or populations based on their unique fragment patterns.

Taq Polymerase: Taq polymerase is a thermostable DNA polymerase derived from the thermophilic bacterium Thermus aquaticus. It is a crucial enzyme used in the Polymerase Chain Reaction (PCR) for amplifying DNA sequences. Its ability to withstand high temperatures allows it to remain active during the denaturation phase of PCR, making it essential for producing large quantities of specific DNA segments efficiently.

Ubiquitination: Ubiquitination is a post-translational modification process where small proteins called ubiquitins are attached to a target protein. This modification serves various functions, most notably in tagging proteins for degradation via the proteasome, regulating protein activity, and influencing cellular localization. Ubiquitination plays a crucial role in maintaining cellular homeostasis, controlling the cell cycle, and regulating signal transduction pathways. Dysfunction in ubiquitination processes can lead to various diseases, including cancer and neurodegenerative disorders.